Analysis of Technology Options to Reduce the Fuel Consumption of Idling Trucks
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ANL/ESD-43
Analysis of Technology Options to Reduce
the Fuel Consumption of Idling Trucks
Center for Transportation Research
Argonne National Laboratory
Operated by The University of Chicago,
under Contract W-31-109-Eng-38, for the
United States Department of Energy
Argonne National Laboratory
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ANL/ESD-43 Analysis of Technology Options to Reduce the Fuel Consumption of Idling Trucks by Frank Stodolsky, Linda Gaines, and Anant Vyas Center for Transportation Research, Energy Systems Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 June 2000 Work sponsored by the United States Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, Office of Heavy Vehicle Technologies
This report is printed on recycled paper.
Contents
Acknowledgments .................................................................................................................... v
Abstract..................................................................................................................................... 1
1 Introduction ........................................................................................................................ 3
2 Truck Idling........................................................................................................................ 5
3 Technology Options to Reduce Idling ............................................................................... 9
4 Potential Implications of New Technologies ..................................................................... 15
4.1 Reduced Energy Use................................................................................................. 15
4.2 Emissions Reductions ............................................................................................... 17
4.3 Cost Analysis ............................................................................................................ 18
5 Conclusions........................................................................................................................ 21
5.1 National Benefits ...................................................................................................... 21
5.2 Summary ................................................................................................................... 22
6 References.......................................................................................................................... 25
Appendixes
Appendix A: Equipment and Emissions Data ......................................................................... 29
Appendix B: Emissions Data................................................................................................... 31
Figures
1 Typical Tractor-Trailer ................................................................................................... 3
2 Typical Sleeper ............................................................................................................... 6
3 Espar Heater.................................................................................................................... 11
4 Pony Pack Auxiliary Power Unit.................................................................................... 11
5 Webasto Thermocooler®, Auxiliary Climate Control System........................................ 12
iiiTables
1 Estimated Idling Hours for Long-Haul Trucks ............................................................... 7
2 Technology Options for Reducing Overnight Diesel Engine Idling .............................. 10
3 Estimated Cab/Sleeper Heating and Engine Block Heating
Energy Requirements...................................................................................................... 16
4 Estimated Emissions from Truck Cab/Sleeper and Engine Block Heating.................... 18
5 Economic Analysis of Equipment Options, as Function
of Idling Hours Displaced............................................................................................... 20
6 Baseline Annual Heating/Cooling Energy Use and CO2 Emissions
for 100% Market Penetration by Alternative Technologies ........................................... 22
B.1 Idling Emissions.............................................................................................................. 31
B.2 Direct-Fired Heater Emissions........................................................................................ 31
B.3 Power Plant Emissions.................................................................................................... 31
B.4 Pony Pack Emissions ...................................................................................................... 32
ivAcknowledgments
The authors wish to thank Sidney Diamond, Program Manager, Office of Heavy Vehicle
Technologies, and James J. Eberhardt, Director, Office of Heavy Vehicle Technologies. We also
thank the other members of the Center for Transportation Research who helped with this study:
Rolando (Roy) Cuenca, Kenneth Wilund, Margaret Singh, and Michael Wang. In addition, we
thank Jules Routbort for facilitating contacts among the parties involved. The authors also thank
equipment vendors, trucking companies, and trucking organizations for their constructive
comments on the earlier draft.
vvi
Abstract
Long-haul trucks idling overnight consume more than 838 million gallons
(20 million barrels) of fuel annually. Idling also emits pollutants. Truck drivers
idle their engines primarily to (1) heat or cool the cab and/or sleeper, (2) keep
the fuel warm in winter, and (3) keep the engine warm in the winter so that the
engine is easier to start. Alternatives to overnight idling could save much of this
fuel, reduce emissions, and cut operating costs. Several fuel-efficient alternatives
to idling are available to provide heating and cooling: (1) direct-fired heater for
cab/sleeper heating, with or without storage cooling; (2) auxiliary power units;
and (3) truck stop electrification. Many of these technologies have drawbacks
that limit market acceptance. Options that supply electricity are economically
viable for trucks that are idled for 1,000–3,000 or more hours a year, while
heater units could be used across the board. Payback times for fleets, which
would receive quantity discounts on the prices, would be somewhat shorter.
12
Section 1
Introduction
Intercity tractor-trailers (like the one shown in Figure 1) and other vehicles with diesel
engines idle a significant portion of the time. A typical intercity tractor-trailer idles an estimated
1,830 h/yr when parked overnight at truck stops. Nationally, a significant amount of fuel is
consumed this way — trucks that travel more than 500 mi/d consume about 838 million gal of
diesel fuel annually during idling. Drivers have many reasons for keeping the diesel engine
running in a tractor-trailer: (1) to keep the cab and/or sleeper heated or cooled, (2) to keep the
fuel warm in winter, (3) to keep the engine warm in the winter to permit easier startup, and
(4) because all the other drivers do it (LaBelle 1986). Other trucks and vehicles with diesel
engines are also idled for long periods: school bus drivers idle their buses in the morning to
defrost the windshield and heat the bus, and transit bus drivers idle their buses to heat or cool the
bus while waiting to pick up passengers at terminals (Jessiman 1996). Off-highway vehicles and
locomotives are idled to keep the engine and fuel warm in cold weather.
Idling also produces airborne emissions and noise; a number of cities and municipalities
have banned or restricted idling to reduce pollution and noise. For example, Philadelphia bans
idling of heavy-duty diesel-powered motor vehicles (City of Philadelphia 1986). Exceptions are
made during cold weather. Idling is limited to up to 5 consecutive minutes when the ambient
temperature is less than 32°F and up to 20 minutes when the ambient temperature is less than
20°F. Buses may be idled up to 20 consecutive minutes if they are equipped with air-
conditioning and the ambient temperature is 75°F or greater.
Various off-the-shelf technologies are available to heat and cool the sleeper and cab of a
truck and to keep the engine and fuel warm in the winter without idling the engine. However,
many of these technologies have draw-
backs that limit market acceptance.
Other technologies are less mature but
promise to eliminate the drawbacks of
the current technologies. This report
presents a brief overview of (1) the
extent of truck diesel engine idling,
(2) technology options to reduce truck
diesel engine idling, (3) estimated
energy and emissions impacts of each
technology option, and (4) the estimated
cost of each technology option. A more
detailed analysis of technology options,
costs, and benefits is recommended,
Figure 1 Typical Tractor-Trailer
given the potentially significant energy
savings and emissions benefits from
reducing truck diesel engine idling.
34
Section 2
Truck Idling
The 1997 Vehicle Inventory and Use Survey reports that 2.54 million trucks had a
manufacturer’s gross vehicle weight (GVW) rating of 26,001 lb or greater, and these trucks are
classified as class 8 trucks (VIUS 2000). They may be single-unit trucks or combination trucks
(single-unit trucks with a trailer or tractor-trucks with single, double, or triple trailers). Seventy
percent of these trucks are operated within 200 mi from the home base, and 83% are operated
within 500 mi of home base. However, about 458,000 combination trucks travel more than
500 mi from their home base each day. Most, but not all, of these trucks are equipped with
sleepers. These are the trucks that are likely to be idling overnight during stopovers on long trips.
Drivers idle their truck engines to keep the cab and sleeper area warm when they are
sleeping or resting in the truck (or away but likely to return soon); in extremely cold weather,
drivers idle their truck engines to keep fuel from freezing and to keep the engine block warm to
prevent difficult starting and smoke. They also idle the engines to run air-conditioning during hot
weather and to provide power for appliances and other electrical devices. Sleepers can be quite
comfortable and well equipped with modern conveniences, such as personal computers, stereos,
and televisions. Figure 2 shows a typical sleeper compartment.
Detailed data on idling are not available. However, industry sources have provided rough
estimates. One source reports that a long-distance, freight-hauling, heavy-duty truck idles about
6 h/d on the average (TMC 1995). The actual extent of idling varies with the season and with the
type of operation. A truck may idle for 10 h/d during winter and less than 5 h/d the rest of the
year; the baseline entry in Table 1 reflects a 6-h/d average,1 with a seasonal peak in the winter.
Another source estimates that the average is higher 40% of the time is spent idling on the
basis of information provided by J.B. Hunt (Whiteside 1996). Most trucking organizations now
offer incentives to reduce idling, and some large firms have succeeded in reducing truck idling
time to 20% (Whiteside 1996). However, nearly 40% of the long-haul trucks are in fleets of less
than 25 vehicles, and these small fleets are less likely to have such incentive programs.
We interviewed several truck operators and fleet owners and found a wide variation in
actual behavior. At one extreme, one owner-operator of an older truck reported that he leaves his
truck running all the time, even at home over the weekend, to make absolutely sure that it will
start. On an annual basis, he idles his truck for more than 5,000 h. At the other extreme is another
owner-operator, who lives in Minnesota, makes two round trips to Chicago each week, and only
runs his truck when he is in it. He plugs in a small electric heater to keep the engine warm at
home in the winter, but he still does occasionally have trouble starting the truck. He idles his
truck for fewer than 1,000 h/yr. In between these extremes, an Iowa contract fleet owner reports
that his drivers run their trucks all the time during the week, but the trucks are turned off for the
1 Eighty-five winter days at 10 h/d and 218 nonwinter days at 4.5 h/d.
5weekend in his yard (Riemer 1999). These
40 trucks are idling for approximately 3,600 h
annually. In the absence of actual statistical
data, we present results as a function of
number of hours idled per year. Table 1 shows
estimates for total numbers of long-haul truck-
hours idling annually in the United States.
The location and the availability of
appropriate places to stop also influence truck
idling patterns. Truck drivers stop to rest at
public rest areas and private truck stops
(TRI 1996). Most truck drivers use free public
rest areas during the day and for short stops.
Nearly 1,500 public rest areas are scattered
throughout the contiguous 48 states and the
District of Columbia, offering some
25,000 truck parking spaces for the nearly
480,000 long-distance trucks (including those
that do not need to park overnight on the road).
Accessibility is an important concern, which
favors the public rest areas on interstate routes.
Therefore, the public facilities along the busy
interstate routes (such as I-5, I-10, I-40, I-80,
I-90, and I-95) become full early (TRI 1996).
Private truck stops provide approximately
184,000 additional parking spaces, and many
truck drivers use these, with new construction
Figure 2 Typical Sleeper projected to increase that number to 213,000
by the end of 1999 (TRI 1996). Although
many trucker drivers prefer not to pay user
fees, higher fuel taxes, or higher registration fees for parking, truck stops offer such services as
showers, laundry, and restaurants (TRI 1996). The truck stops along the busy interstate routes
report 90% occupancy levels. Drivers sometimes park at their customers’ lots if the site is safe.
Drivers must balance their concerns for safety and cost in selecting a place to rest.
If there are 458,000 truck drivers who are likely to need to stop and sleep, and there are only
about 210,000 parking spaces at rest areas and truck stops, then a maximum of 46% of the heavy
long-haul trucks can be using them at any one time. Although trucks are out of service part of the
time for maintenance, there is peak utilization on weekdays, which increases the overcrowding.
Many truckers drive at night and sleep during the day.
6Table 1 Estimated Idling Hours for
Long-Haul Trucks
Truck-Hours
Average Hours per per Year
Hours per Day Year (millions)
3.3 1,000 458
6 (base case) 1,830 838
9.9 3,000 1,374
16.5 5,000 2,290
78
Section 3
Technology Options
to Reduce Idling
Although idling patterns could be changed via regulation or other incentives to change
behavior, these options are not considered here. Several technological alternatives could be used
anywhere to displace diesel engine idling and provide truck heating and cooling.2 These options
include (1) direct-fired heaters (heating only), (2) thermal storage systems, and (3) auxiliary
power units (APUs). In addition, electrification of truck stops to provide parked vehicles with
electricity for heating, cooling, and other purposes has been proposed (EEI 1996), but that option
is not commercial. Each technology is at a different level of maturity, and each technology has a
number of variations. (Table 2 summarizes the technology options and the benefits and
drawbacks of each.) Although several manufacturers are mentioned in this section, our
equipment survey was by no means complete, and no operational tests were performed.
Therefore, no endorsements are to be inferred. The units described here are generally suitable for
retrofitting on an operating truck. Some are already available as factory options on new trucks as
well.
Direct-fired heaters can be used to heat both the cab/sleeper and engine or just one or the
other (see Figure 3). Units are commercially available from a number of manufacturers. One of
the largest suppliers in North America is Espar Heater Systems, the parent company of which,
Eberspächer, has been in the business making heaters for vehicles since the 1930s
(Eberspächer 1990). Espar’s first direct-fired heater was developed in the early 1950s. Another
company, Webasto, also makes several models for trucks and buses. Direct-fired heaters use
much less fuel than the diesel engine to provide heating because they supply heat directly from a
combustion flame to a small heat exchanger. The diesel engine must first burn fuel to overcome
engine friction, and only part of the waste heat from the engine is transferred to the heating
system.
The market share of direct-fired heaters is relatively low because of safety concerns,
retrofitting costs, and unknown reliability (Whiteside 1996). For example, during cold weather,
some truck drivers express concern about drain on the battery by the heater. Although this
concern does not appear to be valid, the Detroit Diesel Corporation (DDC) markets an
electronically controlled optimized idling device that monitors engine temperature and battery
voltage and automatically starts and stops the engine when necessary to maintain battery charge
or to keep engine temperatures from dropping so low that starting might be difficult. However,
noise from engine starting and stopping, using such devices, was reported to wake up sleeping
drivers (Riemer 1999). A gradual start/stop has been proposed to alleviate this concern. DDC
also markets an optional thermostat to monitor the temperature in the cab/sleeper (DDC 1995).
2 We do not consider timers because they are designed to manage engine operation rather than provide
heating and cooling.
9Table 2 Technology Options for Reducing Overnight Diesel Engine Idlinga
Technology
Technology Function Benefits Drawbacks Status
Direct-fired heater Heating for Can be used at any stop Cannot provide cooling. Commercial.
cab/sleeper and/or for heating. Small and Requires battery power
engine. lightweight. and may be unreliable
when not equipped with
automatic engine starting.b
Auxiliary power unit Heating and air- Can be used at any stop Heavier and larger than Commercial.
conditioning of for heating, cooling, and direct-fired heater. May
cab/sleeper, heat auxiliaries. Recovers require separate sleeper
for engine, and waste heat for space air conditioner.
power for heating. Serves as
auxiliaries. survival system.
Thermal storage Heating and air- Driver comfort. Does not heat engine. At or near-
conditioning for Requires relatively large commercial.
cab/sleeper only. space for storage medium. Commercial in other
Performance dependent applications.
on truck use.
Direct heat with thermal Heating and air- Can be used at any stop Requires battery power. Commercial.
storage cooling conditioning of for heating and cooling.
cab/sleeper and
heat for engine.
Truck stop electrification Provides electricity Provides power for heating Limited choice of over- Not commercial.
for heating, air- and cooling and night location. Requires Commercial in other
conditioning, and auxiliaries. separate sleeper air applications.
auxiliaries. conditioner and electrically
powered heater. Requires
infrastructure at the truck
stop.
a
Excludes timer devices.
b
A thermoelectric device is being developed by Hi-Z Technology that promises to reduce the need for battery power
(HiZ 1996).
References: Direct-fired heater: Espar (undated); Thermal storage: PCG (1995).
In addition, thermoelectric devices that reduce the battery power needed by direct-fired
heaters are in the research and development stage (HiZ 1996). HiZ Technology proposes to
modify its current-production 14-W and 24-W units for operation on direct-fired heaters.
Thermoelectric devices convert heat directly to electricity. They would be installed on the hot
end of the direct-fired heaters and provide some power for the cab/sleeper heater. However,
thermoelectric devices are not yet commercial and do not totally eliminate battery use.
Auxiliary power units (APUs) are mounted externally on the truck cab or sleeper and consist
of a small internal combustion engine (usually diesel) equipped with a generator and heat
recovery to provide electricity and heat (see Figure 4). For air-conditioning, an electrically
powered air-conditioner unit is normally installed in the sleeper area, although some units use the
10Figure 3 Espar Heater (Courtesy of Espar Heater Systems)
existing air-conditioning system. APUs are
proven technology. For example, the “Napsac”
from Advanced Thermodynamics Corporation
(ATC) uses an 11-hp, 2-cylinder Detroit Diesel
engine that produces 3.5 kW of AC power at
115 V (ATC undated a). Cab/sleeper heat is
provided by an electric heater mounted in the
unit, and the engine coolant is circulated in the
same unit for additional heat. Another unit by
APC is equipped with a water brake to convert
mechanical (shaft) power to heat (up to
60,000 Btu/h; ATC undated c). A competing
unit, called the Pony Pack, is a 12-V unit (with
inverters available for 110-V needs). It uses a
2-cylinder diesel engine, an alternator, and an
air-conditioning compressor, all within an
Figure 4 Pony Pack Auxiliary Power Unit aluminum frame, that has been integrated with
(Reproduced with permission, Pony Pack, the existing water, fuel, and electrical systems.
Inc.) This commercial unit, which is available as an
option on some new trucks, keeps the battery
and engine coolant warm and cools the cabin, so drivers avoid start-up problems. Manufacturer’s
data on energy consumption and costs for this APU are presented in Appendix A. Actual test
data will be collected from a demonstration fleet of about 10 trucks in Texas (MSR 1999). One
advantage cited for APUs is that they can serve as survival systems in case of truck breakdown in
extreme weather conditions.
In the future, it might be possible to use a gasoline or diesel-powered solid oxide fuel cell
(SOFC) as the power source in an APU. BMW is currently doing research on an SOFC to replace
a car’s battery and alternator (Neff 1999). DOE’s Office of Transportation Technologies is
investigating the feasibility of an SOFC as an APU in trucks. No data are available at this early
concept stage.
11Thermal storage systems (TESs) for trucks are relatively new. A TES consists of a phase-
change material that stores heating or cooling energy that is transferred from the vehicle engine
or air-conditioning while the vehicle is operating (PCG 1995). One company, SHAPE Energy
Resources (Store Heat and Produce Energy, Inc.), offered systems with thermal tank capacities
from 16 to 35 gal, which allow heat storage capacities of up to about 23,000 Btu.
SHAPE Energy Resources is now out of business. With TESs, heating and cooling, but no
electrical power, can be supplied to the sleeper compartment (only) for up to 8 h while the cab is
allowed to cool off or heat up. A small amount of electrical power is required. TESs are large
compared with the direct-fired heaters (which only supply heat), but they are similar in size to
APUs. These units have been tested by several large trucking companies, including Schneider
and Dart Transit, and have been evaluated by Peterbilt and others as factory options
(SHAPE 1997). The effectiveness of TESs depends on the duty cycle of the truck, which could
be a disadvantage under certain conditions (for example, if the air conditioner was not used
during the day, the TES could not cool the sleeper overnight). Limited cost and performance data
are available for this technology. The Thermocooler, a variant offered by Webasto (see Figure 5),
®
Figure 5 Webasto Thermocooler , Auxiliary Climate Control System
(Reproduced with permission, Webasto Thermosystems, Inc., Lapeer,
Michigan)
12combines a direct-fired heater with a thermal storage unit for cooling and is claimed to have
sufficient heat to keep both the engine and the cab/sleeper warm (Webasto 1999).
Truck stop electrification is another option for reducing truck idling. The trucker would
simply “plug in” the truck to outlets at the truck stop to power heaters, air conditioners, marker
lights, and accessories like microwave ovens and refrigerators. Electrification involves
modifying the truck stop as well as the truck. Currently, there are no truck stops that provide
plug-in power for truck and other heavy-vehicle needs, but a pilot test is planned as part of the
Texas Clean Cities program (MSR 1999). Also, the South Coast Air Quality Management
District (SCAQMD) has a program for truck stop operators to generate mobile source emission
reduction credits by providing electrification for trucks (SCAQMD 1997).
Electrification involves the installation at truck stops of ground electric outlets (or plates in
case of induction) at each parking space. It also involves retrofitting trucks with an electric
engine block heater, an electric fuel heater, an electric heating/cooling device for cab and sleeper
conditioning, and electric automatic idle control. A relay to bypass the battery and activate the
cab’s electric system is also included. Provisions for buses and electric vehicles are also possible.
Truckers will not install the necessary equipment when there is no place to plug it in, and truck
stop owners will not install infrastructure when nobody has the equipment to use it. Volvo is
reported to offer an electrification option on new trucks (employing AC power systems) (T-SEA
1998), and Freightliner will soon as well. In addition, Volvo has purchased an interest in a chain
of truck stops (Tempchin 1999), which they might electrify.
As part of engine manufacturers’ efforts to reduce parasitic power in their products, they are
replacing shaft-driven pumps and accessories with electrically driven devices. Electrically driven
pumps and compressors, such as those marketed by Sandon International, can be operated only
on demand, thereby reducing the overall parasitic load on the engine (Brooks 1999). The industry
envisions that the “electrified” truck will be capable of being plugged into outlets at the truck
stop. Specific Climate Systems (SCS/Frigette) produces several types of 110-V, electrically
driven cooling and heating systems that can be either be plugged in or operated by the engine’s
alternator. SCS/Frigette also produces deep-cycle battery packs that provide 864 Ah, which
would provide an estimated 8 h of air-conditioning or heating (Pannell 1999).
1314
Section 4
Potential Implications
of New Technologies
4.1 Reduced Energy Use
Fuel consumption is reduced by eliminating unnecessary engine idling. Table 3 provides
data on (1) the energy used during engine idling when providing heating and (2) the energy
required for several alternatives to idling, including truck stop electrification. Note that the heater
and storage systems are not directly comparable with the other alternatives because they do not
provide the full range of services. Appendix B contains more detailed information about
emissions data, while Appendix A provides equipment and emissions data. About 10,650 Btu/h
is needed to keep the engine warm, which is done by warming the coolant, and an additional
~4,100 Btu/h is needed to provide cab/sleeper heating. Heating can be supplied by circulating the
coolant if it is being warmed or by providing a separate air heater. (The engine requires more
heat because it is not insulated.) These requirements are based on medium settings for the Espar
heating system. More heat is provided at higher settings with additional fuel consumption; other
units have similar capacities. Energy requirements can be reduced by better insulating the truck
and by using sunshades during daytime stops. Insufficient data were available to do a complete
analysis of cooling systems; therefore, most of the discussion here focuses on heating.
The idling fuel-use rate varies, depending on equipment load and engine speed. To avoid
engine wear due to low-speed idling, most truckers idle their engines at 1,000 rpm with some
load. The Truck Maintenance Council (TMC) suggests fuel flow rates for various loads and
engine speeds in its Recommended Practice Bulletin 1108. Diesel engine idling consumes about
one gallon of diesel fuel per hour when the truck’s heating or air-conditioning system is operated
(for 5 brake horsepower load at 1,000 rpm; TMC 1995), or about 128,500 Btu/h (the newest
engines consume slightly less). Note that the engine is only about 11% efficient when used to
heat the cab/sleeper and the block (14,750 Btu/h output [cab/sleeper heater plus engine block
heater] divided by 128,500 Btu/h fuel consumption).
The engine provides cab and engine block heating as a by-product of producing shaft work,
but a directly fired heater avoids the losses associated with the production of shaft work by
simply indirectly heating the air that enters the cab/sleeper area. A typical furnace efficiency is
80−85%. Therefore, the direct-fired heater for providing cab/sleeper and engine heat is extremely
efficient compared with engine idling or electric heating. Assuming that the battery energy
consumed during heater use must be supplied by the engine after restart, the energy requirement
is about 18,540 Btu/h, yielding an overall efficiency of 80%. Efficiency of the APUs is high as
well, because the engine is sized to meet electricity and heat requirements.
Data were not available to evaluate energy consumption by the storage systems. Product
literature states that thermal storage units require energy (~50 W) for operating fans and
15Table 3 Estimated Cab/Sleeper Heating and Engine Block Heating Energy
Requirementsa
Electrical
Power Total Usable Net
Fuel Input Required or Energy Output Efficiencyb
Option (Btu/h) Produced (W) (Btu/h) (%)
Truck engine idlingc 128,500 1,300 19,187 15 (11 for heat
only)
Direct-fired heaterc 18,438 52 14,750 80
Auxiliary power unit 23,130 1,300 19,187 83 (64 for heat
only)
Thermal storage (cab heat only)d – 30 102 –
Truck stop electrification 45,378 4,300 14,676 33
a To provide 10,650 Btu/h to heat engine block and 4,100 Btu/h to heat cab/sleeper, based on medium
setting of direct-fired heater.
b Defined as energy output (as heat to provide warming of engine and cab/sleeper) divided by energy input.
c Engine idling provides more heat than is required to maintain acceptable low-smoke start-up
temperatures. We assume 10,650 Btu/h is required to maintain acceptable start-up temperature.
d Includes energy required to supply electricity, including that to recharge partially discharged batteries.
References: EEI (1996); Espar (undated); Wang (1996); TMC (1995).
pumping the storage medium while the unit is running. Manufacturers do not provide information
on the increased fuel requirements during vehicle operation to supply additional heating or
cooling for the storage medium. However, Webasto estimates an increased load of 1 hp to cool
the medium (Koziel 1999). If the truck averages 200 hp at 55 mph and gets 6 mpg, it uses
37−55 gal of fuel during the 4−6 h that the medium is cooled. The marginal cooling load adds
about 0.5% (1/200) to that, or 0.19−0.28 gal. This is the fuel used for 8 h of cooling, so the fuel
use is approximately 0.023−0.034 gal/h of cooling.
One drawback of the direct-fired heater and storage options is that they need electrical
power, and so truckers are concerned that they will not have enough battery capacity to restart
the engine, especially during cold weather (Jessiman 1996). During a typical overnight heating
cycle of 8 h, at a current draw of 1.9 A, a cab/sleeper heater requires about 15 Ah of battery
capacity (Espar undated). The engine block heater requires power for the coolant circulating
pump; about 3 A is required for this unit on the medium setting (5.8 A on high), and so about
24 Ah of battery capacity is required overnight on the medium setting (46 Ah on high). A total
battery capacity of 39 Ah is therefore required on the medium setting (61 on high). The typical
truck has three 12-V batteries with a total capacity of 150 Ah, and engine start-up requires about
60 Ah. Under most conditions, sufficient battery capacity would therefore remain for start-up. In
addition, thermoelectric converters for direct-fired heaters promise to reduce electrical power
16requirements (Hi-Z 1996). A 12-W unit could provide approximately 8 Ah, but it would not
totally eliminate the need for battery power.
The results of a literature review of APUs suggest that about 23,000−40,000 Btu/h of
primary energy is required for heating and cooling, which is considerably less energy than the
128,500 Btu/h used when the truck idles (ATC undated a; ATC undated b; Greer 1999).
If electrical power supplies the energy for cab/sleeper heating and engine block heating,
about 4.3 kW is required to supply the assumed 14,750 Btu/h requirement. The primary energy
consumed at the power plant is 45,378 Btu/h, assuming a conversion efficiency of 32.5%. Note
that very little of the primary energy used to generate electricity is in the form of petroleum, so
electrification minimizes oil use.
4.2 Emissions Reductions
Using more efficient alternatives to diesel engine idling also reduces emissions. Estimated
emissions for current practice and alternatives are listed in Table 4. No data are available for
storage options, which produce no emissions during their operation, but they are expected to
cause a small increase in truck operating emissions because of increased load on the heating,
ventilation, and air-conditioning (HVAC) system to heat or cool the thermal storage medium.
The data on equipment were provided by the suppliers and should be considered preliminary.
Emissions of carbon dioxide were calculated on the basis of the fuel input. Emissions from
electricity generation are based on the current average U.S. generation mix. Emissions would
differ somewhat by region or time of day (truck drivers sleeping at truck stops at night would be
using off-peak power, which removes gas and oil from the generating mix). Further, more
detailed work could examine the implications of changing the generating mix. With the caveat
that additional, more reliable data are needed, we can make some tentative observations.
Truck engine idling produces significantly higher greenhouse gas (CO2) emissions than any
of the other options considered (because of its higher energy use) and, in almost all cases, emits
more of all pollutants. The direct-fired heater and auxiliary power units generate significantly
less CO2 than electricity use, assuming heating loads in each case are equal. Compared with
idling, electricity use significantly reduces all of the emissions tabulated (except possibly SO2;
see Appendix B) for the current U.S. generation mix. Electricity use reduces particulate
emissions compared with all of the other options for which data were available. These findings
are clear.
However, conclusions about any of the other emissions from onboard units are highly
speculative because of the uncertainty in the data reported. The direct-fired heater and APU may
produce more hydrocarbons than electrification because emissions are relatively high during
start-up and as the heater warms up. Although reported emissions from APUs were significantly
higher for THC (total hydrocarbons), CO (carbon monoxide), NOx (nitrogen oxides), and
particulates than those from the direct-fired heater, both sets of data are suspect. Therefore, these
numbers should not be used to support one type of equipment over another.
17Table 4 Estimated Emissions from Truck Cab/Sleeper and Engine
Block Heating (g/h)a
Emission
Technology VOC or THCb CO NOx PM10c CO2
Idlingd 12.6 94.6 56.7 2.57 10,397
Direct-fired heatere 0.174 0.437 0.264 n/if 1,456
(calc.)
Thermal storageg n/if n/if 4.0 0.10 356
(calc.)
Auxiliary power unith 0.45 7.5 11.6 0.69 1,871
(calc.)
Electrificationi 0.054 0.481 6.04 0.035 3,014
a Data are not comparable; accuracy in doubt.
b Volatile Organic Compounds or total hydrocarbons.
c Particulate matter with Sauter mean diameter of 10 µm and less.
d ATA (1998).
e Espar (1993). From combustion of fuel in the heaters only; excludes combustion of additional
fuel to recharge battery because the amount of fuel used is negligible (584 Btu/h, or 0.036 gal
over an 8-h cycle). Accuracy in doubt.
f n/i = no information.
g Assumes 5 hp-h required to charge the system for 8 h of cooling under full engine load, at a
fuel consumption rate of 169 g/bhph (brake horsepower per hour; calculated on the basis of
average model-year 1996 weight-class 8B truck-specific fuel consumption [EPA 1998]). NOx
and PM emitted at the 1998−2003 model-year-regulated rate of 4 g/bhph NOx and
0.10 g/bhph PM10 (EPA 1997).
h Pony Pack 104, winter operation. Data from Pony Pack, Inc. (undated).
i Providing 4.3 kW to truck heaters. Emissions data from Wang (1999) and DOE (1993).
4.3 Cost Analysis
Cost estimates for several of the technology options were available, and these allow us to do
a simple economic analysis of the options available to reduce overnight idling. Equipment costs
range from about $1,400 installed for a small cab heater to more than $7,000 for an APU with an
inverter to allow use of 110-V appliances. Truck owners could be deterred by the high initial cost
of some of the units, especially the APUs. However, to alleviate this concern, some equipment
manufacturers offer their units for lease, with the option to buy after three years. The capital cost
for the infrastructure for truck electrification is also high and could not be alleviated by leasing
arrangements.3 Another economic concern is the weight of the units that supply cooling.
Although carrying this extra weight would make a negligible difference in the vehicle’s fuel
economy (~300 lb out of 80,000 lb is less than 0.4%), it would reduce the load that could be
carried for trucks that are weight-limited (weight out before they cube out). The extra weight
3 A report by the Truck Research Institute (TRI 1996) suggests a cost of $256−341 million for an increase
of 15,032 spaces in the top-10 critical corridors, or about $17,000−22,700 per space.
18could reduce revenues by as much as 0.6%. For trucks that are volume-limited (“cube out”), there
would be no such penalty.
If idling uses 1 gal/h of diesel fuel and an alternative uses only 0.20 gal/h, at the current
price of about $1.75/gal, the immediate savings in operating cost is $1.40/h, for an annual dollar
savings equal to the number of idling hours avoided per year times 1.4. If there were no other
savings, a $6,000 unit would then have a 17-month payback time, if 3,000 hours of idling were
avoided per year. But fuel savings are not the only cost savings attributable to the reduction of
truck idling. Idling causes engine wear, and reducing idling allows an increase in the interval
between expensive oil changes and lengthens the actual mileage traveled before a truck engine is
likely to need an overhaul. The Truck Maintenance Council (TMC) published a bulletin to enable
estimation of the monetary value of these increases. Using the TMC’s method, we estimate
approximately $0.07 savings on preventive maintenance (oil changes) and $0.07 savings by
increasing the time until overhaul, per hour of idling avoided. These savings are relatively small,
but not insignificant.4
The issue of payback time is an important consideration when examining equipment
purchases. The ATA (1997) has reported that truck owners are only interested in items with a
payback time of under two years. One reason for that is the rapid replacement rates in fleet
trucks. Because reliability is considered so critical, a typical small-fleet owner replaces his or her
trucks every three to four years, which is around the time when warranties expire. In addition, if
all trucks are under warranty, the fleet operation is simplified because the owner does not need a
complete maintenance shop. Larger fleets may be replaced even more often (every two to three
years); taking advantage of volume discounts offered by dealers. With such rapid fleet turnover,
factory-installed units, which are likely to be available at lower cost, could be a more promising
market than retrofits. Retrofits might be attractive for owner-operators who keep their trucks
longer, but small operators often follow larger ones in adoption of any innovation (Riemer 1999).
A simple economic analysis of several options to reduce truck idling is presented in Table 5,
with payback time estimated as a function of hours idled per year, on the basis of the expected
fuel costs for each unit. Savings from reduced maintenance and wear are included. Using this
table requires some caveats. First, as with the emissions data, some of the numbers are projected
because there is no real experience with these options. For instance, the cost for thermal storage
is a target that has not been achieved, and the electrification costs are quite uncertain. Second,
heaters and APUs from different manufacturers have different costs and energy requirements.
Third, the cost of additional fuel to operate the truck on the road if the thermal storage medium
must be heated or cooled has not been included because it is unknown. And finally, for the units
that supply heating only, the number of idling hours includes those occurring during the winter
4 Note that previous estimates of maintenance savings were considerably higher. The difference is in the
number of miles to which one hour of idling is equivalent. The TMC bulletin assesses engine wear
proportional to fuel consumed, so 1 h of idling = 7 mi if the truck gets 7 mpg on the road. Older estimates
equated 1 h to 80 mi, which is twice the wear per hour as driving. Reduction in fuel sulfur (which caused
significant damage to the engine) and change in idling revolutions per minute both contributed to the
change.
19Table 5 Economic Analysis of Equipment Options, as Function of Idling
Hours Displaced
Payback (yr) a
Technology Model Cost ($) @1,000 h @3,000 h
Direct-fired heater D5W + D1L-C 3,200b 1.8 0.60
Thermal storage 2,700c 1.4 0.45
Direct heat with storage cooling Thermocooler 4,200d 2.7 0.93
Auxiliary power unit Power Pak 7,095e 4.3 1.4
Electrification Unspecified 1,700 + 2,500/spot 2.7 0.93
a $1.75/gal diesel fuel.
b Jessiman (1996).
c Estimate from SHAPE (1997).
d Webasto (1999). Payback calculated assuming 59% of operation heating and 41% cooling.
Increased fuel use during operation not known or included.
e IPS literature.
only. For the units that supply cooling as well, all of the idling hours can be displaced, resulting
in a faster payback. With these caveats in mind, we can conclude that options that supply
electricity as well are economically viable for trucks that are idled for 1,000−3,000 or more hours
a year, while the heater units could be used across the board. Payback times for fleets, which
would receive quantity discounts on the prices, would be somewhat shorter.
Note that the cost for electrification includes an estimated $0.08/kW to pay for the power
used, and the payback time is calculated by assuming that only one truck can use each spot per
day. However, if occupancy were increased to two — one sleeping at night and the other during
the day — the payback time would be reduced to 1.9 yr for 1,000 h/yr idling and to 0.6 yr for
3,000 h.
20Section 5
Conclusions
5.1 National Benefits
By using an idling fuel-consumption rate of 1 gal/h, the estimated annual overnight idling
fuel use (in gallons) is equal to the number of truck idling hours, as shown in the last column in
Table 1. Long-haul trucks idling overnight consume more than 838 million gallons of fuel
(20 million barrels) annually (base-case estimate). Using alternatives to overnight idling could
save much of this fuel and reduce emissions and operation costs. Short-distance trucks and other
heavy-duty vehicles (such as transit buses, intercity buses, school buses, and railroad
locomotives) also idle for various reasons. If these vehicles are included, the estimated fuel
consumption due to idling climbs to 3.2 billion gallons (76 million barrels) annually.5 However,
most of this usage could not be avoided by using the technologies discussed in this report.
Table 6 summarizes the energy use and CO2 emissions impact for each option over the
entire year (heating and cooling). This baseline estimate assumes that the direct-fired heaters are
operated 10 h/d in the winter and that idling is used to provide cab/sleeper cooling for 4.5 h/d in
the summer, with 100% market penetration in the eligible class 7 and class 8 trucks, for each
technology. It represents the upper bound on energy savings benefits for the assumed number of
idling hours actual benefits of each alternative to idling would be lower for lower rates of
market penetration. In particular, note that electrification impacts are estimated by assuming that
all trucks could use electrified stops, and so all trucks and parking spaces would have to be
electrified and the spaces would have to be used by an average of 1.5 trucks per day (because
there are more trucks than parking spaces). Both scenarios are unlikely. Actual impacts would, of
course, depend on the actual number of idling hours. A survey should be undertaken to estimate
actual current idling patterns in the United States.
Several conclusions can be drawn on the basis of the information in this table. First, any of
the listed alternatives has significant potential to reduce all impacts compared to idling overnight.
Even the direct-fired heater, which is only assumed to be used 85 days per year, reduces total
energy use, petroleum use, and CO2 emissions by about 40% over the whole year and by about
85% for the period when it is used. Auxiliary power units reduce energy and petroleum use and
CO2 emissions by more than 80% for the entire year. Electrification could achieve almost 70%
energy savings, reduce CO2 emissions by 74%, and reduce CO emissions and petroleum use by
more than 99%. Energy storage options are also expected to significantly reduce idling impacts.
Data are considered too preliminary to make conclusions about the relative potential to reduce
the emissions. Most pollutants are reduced by using energy-saving alternatives to idling.
5 Data are not available on fuel consumed by off-highway vehicles during idling.
21Table 6 Baseline Annual Heating/Cooling Energy Use and CO2
Emissions for 100% Market Penetration by Alternative
Technologiesa
CO2
Energy Usage Petroleum Emissions
Technology (1012 Btu) (103 gal) (103 tons)
Truck engine idling 107.5 838,140 9,597
Direct-fired heater plus idle cooling 64.7 504,433 5,764
10.2 79,197 907
Direct-fired heater plus thermal
storage cooling
Auxiliary power unit 19.3 150,860 1,727
Truck stop electrification 34.2 2,127 2,504
a Assumes 458,000 trucks, with 85 days of heating at 10 h/d and 218 d with average
cooling at 4.5 h/d. Electrification assumes electrified truck stops (hot-bunked) to allow
usage by all trucks.
5.2 Summary
Truck engine idling consumes significant amounts of fuel to provide heating in the winter
and cooling in the summer. Many fuel-efficient options are available to provide heating and
cooling without resorting to idling. Promising options include (1) direct-fired heater for
cab/sleeper heating, with or without storage cooling; (2) auxiliary power units; and (3) truck stop
electrification. Along with truck stop electrification, electricity-based devices in the truck (for
example, those supplied by SCS/Frigette) would be needed. Each option has benefits and
drawbacks.
The direct-fired heater is very efficient (about 80%, compared with only 11−15% for truck
idling) because it simply indirectly heats the air that enters the cab/sleeper area. Net energy
conversion efficiency of electrification (33%) is lower than that of the direct-fired heater, and the
market for electrification is limited to the available spaces, but a complete range of services is
provided. Electrification would most likely be successful over heavily traveled routes, where a
trucker could be sure of finding an electrified stop when he or she needed it. Auxiliary power
units are very efficient and they provide all necessary services at any location. A drawback of
direct-fired heaters is that they do not provide air-conditioning or electric power. Another
drawback is that they need electrical power, and so truckers will be concerned that they will not
have enough battery capacity to restart the engine, especially during cold weather. This option
has had limited market penetration in the past. But direct-fired heaters are small and relatively
inexpensive, and thermoelectric power-conversion devices are under development that promise
to help reduce their electricity consumption.
If typical trucks average 1,830 h of idling annually, eliminating idling and installing and
operating direct-fired heaters during winter would save about 311 million gallons
22(7.5 million bbl) of diesel fuel (850 h/yr, thus yielding fuel savings of 80%), on the basis of a
truck population of 458,000. This savings is equivalent to about 0.2% of total transportation oil
use and 1% of heavy truck oil use. The investment cost would be about $1 billion, and the annual
fuel savings would be approximately $544 million (at $1.75/gal diesel fuel). A maximum of
about 836 million gallons (19.9 million bbl) of diesel fuel could be displaced annually if all
458,000 trucks were to use electric heating and cooling at electric truck stops (most fuels used
for generation of electricity are not petroleum-based). The actual potential savings depends on
the number of hours of idling displaced. Both of these technologies have other applications
direct-fired heaters can be used on school buses and urban buses, as well as on trucks.
Electrification can service transit buses and electric vehicles. Other technologies could also
enable considerable savings.
Market acceptance of any of the options will depend primarily on the perceived benefit of
the technology. The market for direct-fired heaters in new trucks is about 2% (Jessiman 1996),
current APU sales are believed to be a few hundred per year, and the market for electrification is
extremely small. Therefore, market acceptance of any of these technologies would have to be
augmented by appropriate government incentives. Research and development to make these
devices reliable would also augment market acceptance and would encourage researchers to
explore new technologies, like fuel-cell APUs.
2324
Section 6
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